Upper lethal temperature ranges for two strains of Lahontan cutthroat trout (Salmo clarki henshawi) were 18.5–20.2 C in Pyramid Lake water (bicarbonate alkalinity, 1,487 mg/liter), 20.2–21.1 C in Dunn Hatchery well water (357 mg/liter), and 21.8–23.0 C in Truckee River water (69 mg/liter). Within each lethal temperature range, the Summit Lake strain was slightly more resistant to heat death than the Marble Bluff strain.